FAAE production has been practiced for many years yet the FAAE industry in general and the biodiesel industry in particular must keep innovating in order to remain economically competitive. For instance, to avoid cost premiums compared to petroleum diesel and to minimize public concerns about using edible oils for fuel production, biodiesel producers must adapt to feedstocks such as fatty acid distillates, used cooking oil, animal fats, poultry fats, corn oil, pennycress oil, palm oil, algal oils, or other emerging feedstocks. Additionally, commercial biodiesel quality requirements have continuously tightened as biodiesel use has become more widespread, and FAAE products must now be refined to be sold as fungible biodiesel.
Depending on the source of the raw material and the level of processing or refining, the free fatty acid (FFA) content of FAAE feedstocks may be between 0 and 100% by weight. An economic analysis of typical processes for FAAE or biodiesel production indicates that feedstock cost is the largest portion of production cost for a conventional production facility. Generally, feedstocks with higher FFA content (e.g., greater than about 0.5 wt %) are less expensive and can therefore provide significant economic advantages. However, many FAAE production processes cannot produce commercially acceptable biodiesel from the full range of higher FFA feedstocks since they were not designed to do so.
FFA's in FAAE feedstocks present challenges for refined FAAE production with traditional base-catalyzed transesterification processes that were designed to process glyceride feedstocks (i.e., mono-, di- and triglycerides) with low FFA contents (e.g., less than about 0.5 wt %). In such a process, the FFAs are converted to soaps, leading to yield losses and undesirable processing consequences (e.g., emulsion formation, poor conversion, poor separations, poor product quality, etc.). Enzyme-catalyzed conversion of FFAs and glycerides may avoid soap formation and yield losses in the future, but such processes are not currently economically competitive. Alternatively, a feedstock pretreatment or refining process may be used to reduce and/or convert the FFA in the feedstock so that very little FFA remains, and the refined feedstock can then be processed using a base-catalyzed transesterification process.
One method to remove small amounts of FFA (i.e., up to about 4 wt %) is by adding caustic to convert the FFA to soap which can then be removed from the fat or oil as a “soapstock” stream by water washing, centrifuging, and filtering (or “bleaching”). This approach however is not appropriate for feedstocks containing high quantities of FFA (i.e., more than about 4 wt %). It also creates a yield loss of all of the saponified FFA along with the glycerides that are included in the soapstock stream, which has very little commercial value, and the bleaching filter, which has even less commercial value. Another method to remove FFA in feedstocks is by distillation. This process can concentrate the FFA in a distillate stream to greater than 80 wt % while reducing the FFA level in the remaining feedstock to as low as 0.1 wt % (i.e., to an acid number of ˜0.2 mg KOH/g). However, this process also reduces the overall yield of feedstock to FAAE and generates a stream of concentrated FFA that has less value than FAAE.
Yet another method is to convert FFA directly into FAAE using acid-catalyzed esterification with alcohol. The esterification reaction is affected by many variables, including temperature, molar ratio of alcohol to FFA, mass transfer limitations, catalyst concentration, reaction time, and reaction stoichiometry. Since esterification reactions are reversible, the reaction does not go to completion in a single reaction step. Therefore, these equilibrium-limited reactions must be propelled further by increasing the concentration of the reactants or decreasing the concentration of the products, typically by employing multiple reactors with additional process units for water removal and alcohol and catalyst dosing after each reactor. In addition to the high capital expenses for such a system due to the numerous acid-resistant process units required, acidic esterification catalysts with sufficiently low corrosivity to avoid unacceptable corrosion rates of process equipment, whether homogenous or heterogenous, may prove to be too expensive to allow profitable operation.
Glycerolysis of free fatty acids is still another method to convert FFA in an FAAE feedstock. Under certain conditions, FFA and glycerol can be reacted to form mono-, di- and triglycerides (i.e., glycerides) which can then be used to produce FAAE by transesterification. This combined method has the potential to be an advantageous approach to producing FAAE from feedstocks containing FFA for various reasons, including reduced capital expenses compared to acid-catalyzed esterification and more efficient processing because water (the by-product of both esterification and glycerolysis of FFA) can be removed continuously as a vapor stream in glycerolysis. The ability to remove water continuously avoids the need for the additional process units that are required to remove free and dissolved water with direct esterification with lower monohydric alcohols and thereby saves both capital and operating costs.
One challenge with coupling glycerolysis and transesterification is the production of co-products or waste streams that detract from the refined FAAE yield. For instance, in one embodiment the glycerolysis reaction can take place with vigorous mixing between 390° F. and 460° F. and between about 175 Torr and 225 Torr. Under such conditions a significant amount of the reaction mixture including feedstock, FFA and glycerin can be removed in the vapor stream along with the water with the consequence of reduced FAAE and glycerin yields.
Another undesirable by-product stream can be created as a result of incomplete glycerolysis and transesterification reactions. In theory, with a perfectly balanced reaction, all FFA and glycerin reactants would react to form glycerides and water during glycerolysis, and all glycerides would be converted to FAAEs in transesterification while the crude glycerin co-product stream would contain only glycerin, catalyst, excess alcohol, and possibly water but no FAAE or glycerides. In practice, however, the glycerin stream can also contain fatty acid-containing components (e.g., FFA, soaps, glycerides, and FAAE) which have not been completely converted and/or separated in the process. Therefore, as the crude glycerin stream undergoes a refining process, a concentrated stream of FFA, soap, glycerides, water, alcohol, and FAAE that can be described as secondary light phase (SLP) can be separated from the crude glycerin stream which retains most of the glycerin, alcohol, water and catalyst (or salts from neutralized catalyst). Because the SLP stream cannot easily be separated into its individual components by density or distillation, the unrecovered FFA, glyceride, and FAAE components in the SLP represent a diminished yield of the desired refined FAAE product unless further chemical processing is performed. Every fatty acid-containing component in the feedstock that isn't fully converted to FAAE represents a loss of possible yield. The less effective the initial separation of the crude FAAE stream and crude glycerin stream, the more SLP is created at the expense of refined FAAE.
Another challenge involves removing all unbound (or “free”) glycerin from the FAAE stream. While the majority of free glycerin is separated with the crude glycerin, some residual free glycerin remains in the FAAE stream. The acceptable amount of free glycerin in the refined FAAE depends on the market. To produce a commercially-acceptable biodiesel product, for example, the amount of this free glycerin must be less than about 0.02% by weight. One method for minimizing residual free glycerin in the FAAE stream is to employ a reactive distillation unit. The purpose of the reactive distillation unit is to reduce the free glycerin level in the FAAE stream using heat and vacuum. In addition to removing a portion of the free glycerin directly, the reactive distillation unit causes the remainder of the free glycerin in the FAAE stream to react with FFA and/or FAAE to form glycerides which can be recycled back into the transesterification process after being separated from the FAAE stream. However, the reactive distillation unit may consume a significant amount of the final FAAE product depending on the amount of free glycerin it receives. The amount of residual free glycerin in the FAAE stream can therefore have a direct impact on yield of the desired products of FAAE and glycerin.
Because feedstock costs can exceed two-thirds of the total cost of FAAE production, to be economically profitable the FAAE and biodiesel industries must develop processes for producing high quality products from feedstocks with a range of FFA contents. Furthermore, the production process must maximize refined FAAE yield from these feedstocks containing FFAs by minimizing quantities of lower-value co-product streams without sacrificing FAAE quality.